Underneath, data is scattered across systems with different names and formats. A semantic layer maps those into agreed business terms — "customer," "capability," "outcome" — each with a single definition.
The payoff is trust. When a report and a dashboard both draw on the same semantic definitions, they cannot quietly disagree. For AI, the semantic layer supplies the vocabulary the model reasons with, keeping answers consistent and grounded.
A semantic layer often rests on an ontology, which supplies the formal definitions the layer exposes.